Lyme Disease, 2005

In 2005, 918 confirmed Lyme disease cases (17.9 cases per 100,000 population) were reported (Figure 3). This represents a 10% decrease from the record number of 1,023 (20.0 cases per 100,000 population) in 2004, but is markedly higher than the median number of cases reported annually from 1996 through 2004 (median, 464 cases, range 252 to 1,023). In 2005, an additional 19 cases were classified as probable Lyme disease. Five hundred seventy-one (62%) confirmed case-patients in 2005 were male. The median age of case-patients was 39 years (range, 1 to 90 years). Physician-diagnosed erythema migrans was present in 730 (80%) cases. Two hundred twenty-three (24%) cases had at least one late manifestation of Lyme disease (including 167 with a history of objective joint swelling and 42 with cranial neuritis) and confirmation by a positive Western blot test. Eight (1%) Lyme disease cases in 2005 also had objective evidence of human anaplasmosis, another tick-borne disease transmitted by Ixodes scapularis (deer tick or blacklegged tick). Onsets of illness peaked in July (43% of cases), corresponding to the peak activity of nymphal Ixodes scapularis in mid-May through mid-July.

Three hundred ninety-nine (43%) cases occurred among residents of the Twin Cities metropolitan area. However, only 64 (10%) of 612 case-patients with known exposure likely were exposed to infected I. scapularis in metropolitan counties, primarily Anoka and Washington Counties. Most case-patients either resided in or traveled to endemic counties in east-central Minnesota or western Wisconsin. As in 2004, Crow Wing County continued to have the highest number of Lyme disease case exposures (124 [20%] of 612 cases). Risk for Lyme disease continues to be high in certain counties at the northern and western edges (Becker, Beltrami, Clearwater, Hubbard, and Itasca Counties) and southeastern edge (Houston County) of Minnesota’s endemic area.

For a discussion of the recent increase in tick-borne disease in Minnesota and the distribution of ticks that transmit Lyme disease and other tick-borne diseases, see “Expansion of the Range of Vector-borne Disease in Minnesota” in the March/April 2006 issue (vol. 34, no. 2) of the DCN.